This application relates to in situ recovery of liquid and gaseous products from an in situ oil shale retort.
The term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen" which, upon heating, decomposes to produce hydrocarbon liquid and gaseous products. The formation containing kerogen is called "oil shale" herein and the hydrocarbon liquid product is called "shale oil."
One method for recovering shale oil is to form an in situ retort in a subterranean formation containing oil shale. Oil shale formation within an in situ retort site is fragmented to form a retort containing a fragmented permeable mass of formation particles containing oil shale. Formation particles at the top of the fragmented mass are ignited to establish a combustion zone, and an oxygen-supplying gas, such as air, is introduced to the top of the fragmented mass for sustaining the combustion zone and for advancing the combustion zone downwardly through the fragmented mass. As the combustion zone advances through the fragmented mass, hot processing gas from the combustion zone establishes a retorting zone on the advancing side of the combustion zone. In the retorting zone, kerogen in the formation particles is decomposed to produce shale oil and gaseous products. Thus, a retorting zone moves from top to bottom of the fragmented mass in advance of the combustion zone. The shale oil and gaseous products produced in the retorting zone pass to the bottom of the fragmented mass for collection.
U.S. Pat. Nos. 4,043,595 and 4,043,596 disclose methods for explosively expanding formation containing oil shale to form an in situ oil shale retort. Those patents are incorporated herein by this reference. According to a method disclosed in such patents, an in situ retort is formed by excavating formation to form a columnar void bounded by unfragmented formation having a vertically extending free face, drilling blasting holes adjacent the columnar void and parallel to the free face, loading explosive into the blasting holes, and detonating the explosive. This expands the formation adjacent the columnar void toward the free face so that fragmented formation particles occupy the columnar void and the space in the in situ retort site originally occupied by the expanded shale prior to such explosive expansion. The void fraction in the resulting fragmented mass is determined by the volume of formation removed from the retort site to form void space toward which unfragmented formation remaining in the retort site is explosively expanded, inasmuch as such unfragmented formation is fragmented and expanded to fill such a void space. The original void volume is essentially distributed between the fragmented formation particles in the retort being formed.
In the western United States oil shale deposits occur in generally horizontal beds, and within a given bed there are an extremely large number of generally horizontal deposition layers containing kerogen known as "varves." The kerogen content of the formation is often nonuniformly dispersed throughout a given bed.
The average kerogen content of formation containing oil shale can be determined by a standard "Fischer assay" in which a core sample customarily weighing 100 grams and representing one foot of core is subjected to controlled laboratory analysis. The sample is ground into small particles which are placed in a sealed vessel and subjected to heat at a known rate of temperature rise to measure the kerogen content of the core sample. Kerogen content is usually stated in units of "gallons per ton," referring to the number of gallons of shale oil recoverable from a ton of oil shale heated in the same manner as in the Fischer analysis.
The average kerogen content of formation containing oil shale varies over a broad range from essentially barren shale having no kerogen content up to a kerogen content to about 70 gallons per ton. Localized regions can have even higher kerogen contents, but these are not common. It is often considered uneconomical to retort formation containing oil shale having an average kerogen content of less than about 10 gallons per ton. If the average kerogen content is too low, it can be infeasible to retort in situ using a combustion zone in the fragmented mass since the energy available from burning carbonaceous material is not sufficient to heat the oil shale to retorting temperatures.
Formation containing oil shale that is suitable for in situ retorting can be hundreds of feet thick. Often there are strata of substantial thickness within such formation having significantly different kerogen contents than other strata in the same formation. Thus, for example, in one formation containing oil shale in Colorado that is a few hundred feet thick, the average kerogen content is in the order of about 17 gallons per ton. Within this formation there are strata 10 feet or so thick in which the kerogen content is in excess of 30 gallons per ton. In another portion of the same formation there is a stratum almost 30 feet thick having nearly zero kerogen content. Similar stratification of kerogen content occurs in many formations containing oil shale.
As described above, during at least some methods of forming an in situ oil shale retort a vertically extending columnar void is excavated within the formation. Such a vertical columnar void is necessarily located at elevations similar to the formation to be expanded by fragmenting, which also means that such a void be within a stratum of oil shale rich enough to be economically feasible to retort, i.e., having an average kerogen content of greater than about 20 gallons per ton. The fragmented particles formed during the excavation process forming such a void, therefore, contain relatively rich oil shale. Thus, there is a loss of potentially valuable products in the fragmented particles removed from the formation during the excavation of such a void. In the present processes, such fragmented material excavated during the formation of such a void is either discarded or removed to a separate retort to recover the products therein. It is desirable to have an in situ oil shale retorting process in which relatively rich formation excavated to form a void in the retort site is retained for retorting in situ.